Abstract

The technique of dual modulation Faraday rotation spectroscopy (DM-FRS) has been applied to achieve technical-noise-limited detection of HO2 at the exit of an atmospheric pressure flow reactor. This was implemented by combining direct current modulation at 51 kHz of an external cavity quantum cascade laser system with 610 Hz modulation of the magnetic field generated by a Helmholtz coil. The DM-FRS measurement had a 1.5 times better signal-to-noise ratio than a conventional FRS measurement acquired under identical flow reactor conditions. High harmonic detection of the FRS signal also eliminated the substantial offset associated with electromagnetic interference pickup from the Helmholtz coils that is observed in the conventional FRS spectrum. A noise equivalent angle of 8.9×10−9radHz−1/2 was measured for the DM-FRS measurement, corresponding to a 3σ detection limit for HO2 of 0.35ppmvHz−1/2.

HO2 spectra collected using (a) the AC-FRS or (b) the DM-FRS technique. Both spectra were collected at a flow reactor temperature of 600 K, a residence time of 0.2 s, and a gas mixture composition of 0.005∶0.095∶0.90CH3OCH3:O2:He. The measurement bandwidth of the second lock-in amplifier (LIA2 in Fig. 1) was 1.25 Hz, and the measured in-phase (Channel 1) and quadrature (Channel 2) components of the signals are shown in both cases. The uncrossing angles for the polarization analyzer were 0.5° and 1° for (a) and (b), respectively. Channel 2 (shown in red) corresponds to the background channel with no signal. For both figures the peak signal to the baseline is indicated by vertical black arrows. The dotted horizontal line shown in (a) is the baseline for the AC-FRS measurement that accounts for the large y offset due to EMI pickup in the detection electronics.

Comparison between the experimental DM-FRS spectrum (black trace, Exptl. DM-FRS) from Fig. 2(b) and a calculated DM-FRS spectrum (red trace, Calc. from AC-FRS) obtained by numerical evaluation of the AC-FRS spectrum in Fig. 2(a) using Eqs. (1) and (2) with a Δv˜ value of 0.015cm−1.

Normalized peak-to-baseline signal amplitude for the calculated DM-FRS signal plotted against the modulation amplitude of the laser. The calculated DM-FRS signal was normalized to the peak-to-baseline AC-FRS signal in Fig. 2(a). Dashed lines indicate the simulation data point corresponding to the experimental conditions used in Fig. 2(b).